Photolithography is one of the primary processes used in the manufacture of microelectronic devices (e.g., dies) on semiconductor wafers or micromechanical devices. In photolithography, a stepper or scanner machine exposes photosensitive materials to pattern the design of the features onto the semiconductor wafer's surface. A conventional stepper or scanner machine includes a wafer chuck, an illuminator to project light or other radiation, a lens to focus the light, and a reticle. The reticle can be a glass plate with a layer of chrome having a pattern corresponding to the layout of features that are to be constructed on the wafer. In a typical photolithography process, a wafer having a layer of photoresist material is positioned under the lens in the stepper or scanner machine. Next, the illuminator projects a pattern of light onto a portion of the wafer through the corresponding pattern on the reticle and the lens. The light changes the material characteristic of the exposed areas of the photoresist layer to make the exposed areas more or less susceptible to a developing solution. The stepper or scanner machine then positions another portion of the wafer under the reticle and repeats the exposure operation until the entire wafer has been patterned.
In a subsequent operation, the photoresist layer is typically baked and then developed to create the desired pattern in the resist layer for forming the features. Accordingly, the pattern must be formed in the proper location on the photoresist layer and with very precise dimensions to form very small features of 0.11 μm or less. Errors in photolithography can cause many problems including distorted patterns, misplaced patterns, and other defects. These types of errors can ultimately result in defective microelectronic devices. Moreover, as the feature sizes decrease, slight imperfections or misalignments can result in defects.
A common cause of errors in photolithography is contamination of the wafer chuck in the stepper or scanner machine. More specifically, particles or other contaminants from other processes, such as chemical-mechanical planarization, vapor depositions, etc., may remain on the wafer as it is loaded into the stepper or scanner machine. These contaminants can be left on the support surface of the wafer chuck, which prevents subsequent wafers from being properly positioned in the wafer chuck. For example, FIG. 1 is a schematic side cross-sectional view of a wafer 20, a wafer chuck 50, and a particle P between the wafer 20 and the wafer chuck 50 (shown exaggerated for illustrative purposes). The wafer chuck 50 includes a plurality of holes 54 coupled to a vacuum pump 58 to hold the wafer 20 against the chuck 50. The particle P causes the wafer 20 to deform as the vacuum pump 58 draws the water 20 against the chuck 50. This portion of the wafer 20, for example, can project outward from the wafer chuck 50 a distance T. As the feature sizes decrease, even a very small deformation of the wafer 20 may be greater than the field depth of the lens such that the lens may not properly focus the pattern of light on the area of the wafer 20 over the particle P. Improper focusing on a region of the wafer causes what is known as a focus spot. If focus spots are detected at a consistent location on several wafers, the wafer chuck is likely contaminated and must be cleaned.
One problem with existing stepper and scanner machines is that accessing the wafer chuck for cleaning is difficult and the stepper and scanner machines must be recalibrated after each cleaning. More specifically, the cleaning process includes shutting down the stepper or scanner machine, removing one or more panels of the housing, sliding the wafer chuck out of the housing, and cleaning the chuck manually. After the wafer chuck is manually cleaned, the chuck is repositioned in the stepper or scanner machine and the panels are reattached. However, before the stepper or scanner machine can continue processing wafers, it must be recalibrated. Recalibrating stepper and scanner machines is a difficult and time-consuming process that may also require processing target wafers to ensure the stepper or scanner machine is properly registered. The entire process requires significant downtime and consequently causes a significant reduction in throughput. Accordingly, there exists a need to reduce the downtime required to clean the stepper and/or scanner machine.